In a typical radio communications system, mobile radio communications terminals, sometimes referred to as user equipment units (UEs), communicate via a radio access network (RAN) and other networks like the Internet. The radio access network covers a geographical area divided into cell areas, with each cell area being defined as the radio coverage area of a base station (BS) at a base station site, which in some networks is also called a “NodeB”.
The evolution of mobile radio interface standards is strongly focused on packet access technologies where small data units or packets carry the data over the communication medium and a packet header describes the transferred data. See, for example, S. Keshav, An Engineering Approach to Computer Networking, Addison-Wesley professional computing series, ISBN 0-201-63442-2.
One important requirement for these services is a short Round Trip Time (RTT), which is the time that it takes for a packet to traverse from one machine to another and back again. FIG. 1 illustrates the RTT between a user equipment (UE) and a base station (BS). The reason for this requirement is that many upper layer protocols and applications may be delay-sensitive. The Universal Mobile Telecommunication System (UMTS) standard evolution addresses this requirement by reducing the Transmission Time Interval (TTI), defined as the duration of data transmission where coding and interleaving is performed, from 10, 20, 40 and 80 ms down to 2 ms. In the enhanced dedicated channel (E-DCH), which is the transport uplink channel in Wideband Code Division Multiple Access (WCDMA) High-Speed Uplink Packet Access (HSUPA), the TTI contains one transport block, the transport block size is flexible and indicated by the E-DCH Transport Format Combination Identifier (E-TFCl).
Although a short TTI is generally beneficial for upper layer protocols and applications, there is a downside as well. The reliability of the transferred data is a monotonic (increasing) function of the received energy per information bit, and the received energy per information bit, in turn, depends on the transmission power and the transmission time. Because the transmission of data using 2 ms TTIs requires higher transmission power, and is thus in a transmission power limited situation, that transmitted data is more vulnerable to errors than the data transmitted using 10, 20, 40, or 80 ms TTIs. As a result, it is difficult to ensure the same coverage as older, legacy radio interfaces, e.g., previous UMTS releases. The packet data service coverage is especially limited in the uplink direction because the mobile terminal cannot use as high a transmitter power as the base stations in the network transmitting downlink.
One approach to the coverage problem is to employ retransmission (ReTx) protocols where the receiving side requests packet retransmissions from the transmitting side until the packet is successfully received (or the maximum number of retransmission is reached). See the example ReTx (3) in FIG. 1 where the base station requests a ReTx from the mobile terminal. A further improvement is to combine the retransmission protocol with soft-combining functionality where the receiver does not discard erroneously-received packets, but instead buffers their soft-bit values and combines them with the soft-bits values of the retransmitted packets. This is referred to as Hybrid ARQ (HARQ). Although HARQ retransmissions can help alleviate the above-described coverage problems, the number of HARQ retransmissions cannot be too large, otherwise the need for costly additional base station receiver processing resources increases. Moreover, if a large percentage of active mobile terminals are performing frequent HARQ retransmissions, the cost for the provided packet service increases as does the packet delay in the communication, the latter being particularly undesirable for real-time services such as voice.
With these aspects in mind, “Transmission Time Interval (TTI) bundling” (also known as “autonomous retransmissions”) was suggested for LTE uplink (UL) (see 3GPP Tdoc R1-081103, “Reply LS on Uplink Coverage for LTE”, LS from RAN WG1 to RAN WG2 incorporated by reference into this application) as well as for WCDMA HSUPA (see 3GPP Tdoc R1-081619, “EUL coverage enhancements” incorporated by reference into this application). Each TTI bundle corresponds to a single HARQ process and a single block of data, e.g., a single packet. TTI bundling improves coverage without introducing unacceptable delays due to many HARQ RTTs by allowing the mobile terminal to bundle the first HARQ transmission of a data block/packet with a number (N−1) of consecutive HARQ retransmissions of that same data block/packet, i.e., in total “N” HARQ transmissions, without waiting for a negative HARQ acknowledgement (NAK) before making a next one of the N−1 HARQ transmissions. The underlying assumption is that for a mobile terminal with bad coverage, the required number of HARQ retransmissions is expected to be relatively high, so it is highly likely that the first HARQ transmission and some number of HARQ retransmissions would be NAK'ed anyway. Hence, there is no strong reason to wait for the HARQ ACK/NAK for the first HARQ transmission before starting another HARQ retransmission of that same data block/packet.
If the mobile terminal does not receive a positive HARQ ACK for the TTI bundle of N HARQ transmissions, i.e., for either the entire bundle or for at least one HARQ transmission in the TTI bundle, it may make further HARQ retransmissions (single or bundled) until the mobile terminal receives a positive HARQ ACK or reaches the maximum number of HARQ retransmissions. TTI bundling may only be used for mobile terminals with bad coverage because although TTI bundling improves coverage for the mobile terminal, it may have some drawbacks in terms of user throughput, battery consumption, and system capacity. Hence, it is desirable to control the number of HARQ retransmissions in each TTI bundle.
Often, deteriorating coverage from a serving base station is experienced as the mobile terminal moves from a cell area of the serving base station towards one or more cells serviced by a non-serving base station. FIG. 2 illustrates a cellular communications system with a mobile radio terminal communicating with a serving cell A and a non-serving cell B. The mobile terminal typically monitors broadcast signals from neighboring non-serving base stations and keeps a list of the non-serving base stations with the best signal reception. Handover (either hard or soft) usually occurs at some point with one or more of the non-serving base stations in that list.
Handover presents a problem with respect to TTI bundling. If a non-serving base station involved in a soft handover with the mobile connection (or soon to take over the mobile connection in a hard handover) is not made aware that TTI bundling with N HARQ transmissions per bundle is taking place, then the non-serving base station will likely not properly soft combine the different HARQ transmissions. The result is performance degradation caused by buffer corruption when the non-serving base station tries to combine HARQ transmissions that correspond to different data segments and reduced macro diversity gain when the non-serving base stations cannot contribute to the overall reception performance due to this buffer corruption.